The present case is related to co-pending, commonly owned, concurrently filed U.S. Applications entitled xe2x80x9cImproved Emission Silicate Waveguide Compositions For Extended L-Band and S-band Amplificationxe2x80x9d and xe2x80x9cGermanium Free Silicate Waveguide Compositions For Extended L-Band and S-Band Amplificationxe2x80x9d, both of which are hereby incorporated by reference.
The present invention relates to an optical waveguide suitable for optical amplification at wavelengths in the extended L- and S-bands. In particular, the present invention relates to waveguides having a chemical composition that provides for longer excited-state lifetime and enhanced infrared emission.
High-speed optical telecommunications via optical networks allow for the transfer of extremely large amounts of information through optical signals. As these optical signals travel over long distances or are coupled, manipulated, or directed by optical devices, the signals lose their strength. Signal attenuation may be caused by a number of factors, such as the intrinsic absorption and scattering in the transmission fiber, coupling losses, and bending losses. As a signal becomes weaker, it becomes more difficult to detect and propagate the signal. Eventually, a signal may become so weak that the information is lost.
Optical amplification is a technology that intensifies an optical signal. Optical amplification is a vital part of present-day high-speed optical communications.
Optical amplification is typically performed using devices (amplifiers) that contain a pump laser, a wavelength division multiplexer, isolators, gain shaping gratings, and an active rare-earth-doped optical fiber. The typical wavelength range at which present day optical networksxe2x80x94and optical amplifiersxe2x80x94operate is xcx9c1530-1570 nm, the so-called C-band. A band may be defined as a range of wavelengths, i.e., an operating envelope, within which the optical signals may be handled. A greater number of available bands generally translates into more available communication channels. The more channels, the more information may be transmitted.
Each band is identified with a letter denomination. Band denominations used in the present application are:
Currently, high-speed internet-backbone optical fiber networks rely on optical amplifiers to provide signal enhancement about every 40-100 km. State-of-the-art commercial systems rely on dense wavelength division multiplexing (DWDM) to transmit xcx9c80xe2x88x9210 Gbit/second channels within a narrow wavelength band (e.g. C-band). Channels can be spaced xcx9c0.4 nm apart. These channels can be interleaved with forward and backward transmission (0.4 nm between a forward and backward directed channel) to provide multiterabit/second bidirectional transmission rates over a single fiber.
Recently, with the advent of L-band amplifiers, the optical transmission operating range has been extended from 1530-1565 nm to 1530-1605 nmxe2x80x94using both C- and L-band amplifiers, which provides up to 160 channels/fiber. There is a significant desire for even broader band operation to increase information throughput. Normally operation is limited to a maximum wavelength of xcx9c1605 nm by excited state absorption in the erbium-doped fiber. Operation is theoretically limited to wavelengths of light shorter than xcx9c1650 nm in silicate-based fibers owing to high attenuation from multiphonon absorption at wavelengths greater than 1650 nm. Currently, operation is practically limited to wavelengths shorter than xcx9c1630 nm in a fiber system owing to macrobending losses.
Future systems will potentially use wavelengths from 1450 to 1630 nm, which includes the so-called S-band. Use of the S-band has been demonstrated to nearly double the information carrying capacity of existing two stage C-+L-band systems. Transmissions of up to xcx9c10.5 Tb/s over a single fiber using a C-+L-+S-band configuration have been shown in a laboratory demonstration.
There are generally three approaches to optical amplification in the 1450-1630 nm region; Raman amplification, amplification with rare-earth-doped fiber amplifiers, and amplification that combines Raman and rare-earth-doped components.
Raman Fiber Amplifiers
Raman amplifiers rely on the combination of input photons with lattice vibration (phonons) to shift the pump light to longer wavelengths (Stokes shift). Amplification spectra are broad, but sometimes have unwanted sharp peaks. The process is inefficient, and requires a high power pump source. Such high power pumps include fiber lasers or a series of laser diodes, which can be quite costly. The process is nonlinear with incident intensity. Because it requires high input intensities, the process may lead to other unwanted nonlinear processes such as 4-wave mixing and self phase modulation. Nonetheless, Raman amplifiers are useful in combination with rare-earth-doped amplifiers to increase span lengths, especially for 10 Gbit/s and faster systems operating in the C- and L-bands.
Rare-Earth-Doped Fiber Amplifiers
Rare-earth doped amplifiers rely on excitation of electrons in rare-earth ions by an optical pump and subsequent emission of light as the excited ions relax back to a lower energy state. Excited electrons can relax by two processes: spontaneous emission and stimulated emission. The former leads to unwanted noise and the latter provides amplification. Critical parameters for an amplifier are its spectral breadth, noise figure, and power conversion efficiency (PCE). The latter two parameters correlate with excited state lifetime of the rare-earth ions: longer lifetimes lead to lower noise and higher PCEs. Spectral breadth in the fiber in the C-band, which determines how many channels can be simultaneously amplified in the C-band, correlates with the full-width-half-maximum (FWHM) of the spontaneous emission spectrum of the rare-earth-doped glass.
The majority of commercial amplifiers are based on fibers in which the core glass comprises erbium-doped silicates that contain either aluminum and lanthanum (SALExe2x80x94(silicon, aluminum, lanthanum, erbium)) or aluminum and germanium (SAGE). Of the two traditional fiber types, SAGE provides slightly greater spectral width, which allows for additional channels. SALE fiber generally provides slightly higher solubility of rare earth ions, which enables shorter fibers to be used. This is advantageous to minimize, for example, polarization mode dispersion. SALE and SAGE fibers typically provide amplification in the C- or L-bands, but this leaves a large portion of the low-loss region of the silica transmission fiber unused, namely the S-band and long wavelength portion of the extended L-band region ( greater than 1610 nm).
In the S-band, rare-earth doped fiber amplifiers typically rely on non-silicate thulium (Tm)-doped glasses. Thulium provides a relatively broad emission that is centered at xcx9c1470 nm. The energy levels of thulium are such that multiphonon processes can easily quench this transition, especially in high phonon energy hosts such as silica. For this reason, lower phonon energy glasses such as heavy metal oxides (e.g. germanosilicates) and especially fluoride glasses such as xe2x80x9cZBLANxe2x80x9d are preferred as hosts for the thulium. These non-silicate glasses tend to be difficult to fiberize and splice to existing transmission fiber and to date have limited commercial applications.
In the extended L-band, rare earth doped fibers typically are heavy-metal oxide or fluoride-based. Examples of oxide glasses are those based on tellurium oxide and antimony oxide. Both of these types of glasses are difficult to splice owing to their low melting points and high refractive indices.
In the S- and extended L-band, researchers have worked on an optical amplifier approach using a fiber with a core containing simultaneously erbium and thulium. Unexamined Korean Patent Application; No. 10-1998-00460125 mentions a fiber having a core comprising SiO2, P2O5, Al2O3, GeO2, Er2O3, Tm2O3 (SPAGET). The Er and Tm ions are in the range of 100-3000 ppm and the core can optionally contain Yb, Ho, Pr, and Tb in addition to Er and Tm. The reference further speaks about a cladding that contains SiO2, F, P2O5, and B2O3.
An Erxe2x80x94Tm codoped silica fiber laser has been reported. The laser contained a fiber having a SiO2xe2x80x94Al2O3xe2x80x94GeO2xe2x80x94Er2O3xe2x80x94Tm2O3 core (SAGET) and was pumped at 945-995 nm to obtain emission from Er (xcx9c1.55 xcexcm), Tm (xcx9c1.85-1.96 xcexcm) or both depending upon the parameters of mirrors in the laser cavity, fiber length, pump rate, and pump wavelength. Two fibers were reported. In the first fiber the Er/Tm concentrations were 6000/600 ppm. In the second the concentrations were 1200/6000 ppm. The numerical apertures (NAs) were xcx9c0.27 and xcx9c0.12, respectively. The second mode cutoff was xcx9c1.4 xcexcm in both. The first fiber exhibited lasing (gain), but the second did not.
An amplified spontaneous emission (ASE) light source has been reported that contains Er and Tm and which exhibits significant emission enhancement in the S-band region compared to sources that contain erbium only. The reported fiber contained an SiO2xe2x80x94Al2O3xe2x80x94GeO2xe2x80x94Er2O3xe2x80x94Tm2O3 core (SAGET) and contained two levels of Er/Tm. In the first fiber the Er/Tm concentrations were 1200/6000 ppm. In the second the concentrations were 300/600 ppm. The NAs of the fibers were 0.2 and 0.22 respectively. In both cases an xcx9c90 nm FWHM forward ASE peak was observed from xcx9c1460-1550 nm. The second fiber had an ASE about 5 dB higher than the first.
Finally, L-band amplifier modules have been reported that contain two separate fiber types, one doped only with erbium and one doped only with thulium-doped. The fibers are coupled together. The thulium-doped fiber absorbs a portion of the light emitted from the erbium-doped fiber and modifies the gain slope.
Given the ever increasing demand for broadband services, it is highly desirable to have a single amplifier, compatible with silicate transmission fiber, that has significant gain at wavelengths between 1570 and xcx9c1630 nm, i.e., extended L-band. An extended L-band amplifier operating to xcx9c1630 nm would enable greater than 50% more channels compared to a conventional L-band amplifier. Thus, there is a desire for silicate-based fibers that provide substantial emission in the extended L-band. It is also desirable to have an economical, S-band amplifier that is compatible with the current fiber infrastructure. A desirable fiber amplifier would provide longer lifetime and/or increased emission intensity compared to existing amplifiers along the desired bands.
The present invention is directed to extended L-band amplifiers, and to waveguides and glasses for use in such amplifiers. The present invention further provides desirable S-band emission in a silica-based waveguides and amplifiers.
An optical waveguide according to the present invention comprises a core and a cladding surrounding the core. The cladding comprises a material having a lower refractive index than the core at the operating wavelength of the waveguide. The optical waveguide may be, for example, a laser rod, an optical fiber, or a planar waveguide.
The core comprises silica, Al, a non-fluorescent rare-earth ion, Ge, Er, and Tm. An exemplary non-fluorescent rare-earth ion is La. However, other the non-fluorescent rare-earth ion such as Y, Sc, and Lu also are contemplated.
In exemplary compositions the molar concentrations are: Er from 15 ppm to 3000 ppm, Al from 0.5 mol % to 12 mol %, La less than or equal to 2 mol %, Tm from 15 ppm to 10,000 ppm; and the Ge less than or equal to 15 mol %. The core may further include F. An exemplary concentration of F is less than or equal to 6 anion mol %. In particular embodiments, the concentration of Er is from 150 ppm to 1500 ppm, the concentration of Al is from 4 mol % to 10 mol %, the concentration of Tm is from 150 ppm to 3000 ppm, and/or the concentration of Ge is from 1 mol % to 15 mol %.
In more particular embodiments, the concentration of Al is greater than 1 mol %. The concentration of Al plus Ge plus La may be greater than 5 mol % or 10 mol %. The exemplary concentration of Tm may be greater than 300 ppm or greater than 1000 ppm. In some embodiments, the concentration ratio of Tm/Er is at least 1.
The optical waveguide may have desirable spontaneous emission intensities. Exemplary values are: emission at 1600 nm when pumped at 980 nm of greater than or equal to xe2x88x928 dB relative to the maximum emission intensity at xcx9c1.53 xcexcm; emission at 1650 nm when pumped at 980 nm greater than or equal to xe2x88x9214 dB relative to the maximum emission intensity at xcx9c1.53 xcexcm; emission at 1500 nm when pumped at 980 nm is no less than xe2x88x9210 dB relative to the maximum emission intensity at xcx9c1.53 xcexcm; emission at 1500 nm when pumped at 800 nm is no less than xe2x88x928 dB relative to the maximum emission intensity at xcx9c1.53 xcexcm; and emission at 1500 nm when pumped at 1060 nm is no less than xe2x88x9210 dB relative to the maximum emission intensity at xcx9c1.53 xcexcm.
The average Er3+4I13/2 lifetime of the waveguide may be greater than 1 ms, or preferably greater than 5 ms. The present invention also contemplates optical devices, such as an amplifying device, using the disclosed optical waveguide.
A method of making an erbium-doped optical fiber for use in optical amplifiers according to the present invention comprises the step of providing a substrate tube. High purity silica-based cladding layers are deposited on the inside of the tube. A core glass that comprises silica, Al, a non-fluorescent rare-earth ion, Ge, Er, and Tm is then deposited in the tube. The tube is then collapsed to form a preform. Finally, the preform is drawn to yield optical fiber.
In certain embodiments, the core glass is substantially homogeneous. The core may include at least two regions, wherein one region contains a substantially different Er to Tm ratio than the other region. Said regions may be in an annular arrangement.
The core of such a waveguide may be made with one or more MCVD passes, sol-gel passes or with soot deposition, solution doping, and consolidation passes.